The present invention relates to an ultrasonic imaging system for medical diagnostic purposes, and in particular for an arc scan ultrasonic imaging system having a diverging lens and a path-length compensator which compensates for different amounts of acoustic energy loss encountered as the acoustic beam is angulated at different angles.
Ultrasonic transducer arrays are broadly classified under the categories of linear scan type and sector scan type. Conventional linear scan type arrays comprise piezoelectric transducers, typically 256 in number, which are successively arranged in side by side relation to form a linear array. A group of 16 transducers is selectively activated by delayed burst pulses generated by a commonly shared transmit circuitry so that a focused ultrasonic beam is transmitted. The selected group is successively shifted to the next by at least one transducer element to shift the beam linearly along the array, so that the beam is scanned in a rectangular format. One advantage of the linear scan imaging system relate to the fact that it permits the transmit and receive circuits to be used on a time shared basis. Another advantage is that the linear system provides detailed near-field tomographic images. However, the linear scan system has a disadvantage in that it is incapable of scanning areas behind ribs and the transducer array is itself too bulky for handling.
The transducer array of conventional sector scan systems, on the other hand, usually comprise 32 transducer elements each of which is associated with its own transmit and receive circuitry. The beam is steered in a sector format by the transmit circuit which applies successively delayed pulses. While, the sector scan system is capable of imaging the behind-the-rib areas, the control circuitry is complex and the acoustic beams cluster near the apex of the sector to such a degree that useful tomographic information is not obtained.